A METHOD FOR INCREASING BISULFITE PULP YIELD COMPRISING TREATING LIGNOCELLULOSIC MATERIAL WITH CYANIDE IONS AT A pH OF 7{14 12 PRIOR TO DIGESTION

ABSTRACT

A process for increasing pulp yield from lignocellulosic material consists of treating the lignocellulosic material with either a solution of cyanide ions at a temperature below about 130* C., preferably about 75* C., or treating the material with a mixture of cyanide-bisulfite ions in solution at a temperature below 130* C. preferably about 100* C., and digesting at conventional neutral or acid bisulfite conditions. The treatment of wood prior to neutral or acid bisulfite digestion with either cyanide ion alone or a combination of cyanide-bisulfite results in pulps having increased yield and improved brightness stability over untreated bisulfite cooks.

United States Patent Falkehag et al.

[ 1 May 16, 1972 [54] A METHOD FOR INCREASING BISULFITE PULP YIELDCOMPRISING TREATING LIGNOCELLULOSIC MATERIAL WITH CYANIDE IONS AT A PHOF 7-12 PRIOR TO DIGESTION [72] Inventors: Sten I. Falkehag, Charleston;Alan M.

Bills, Summerville, both of SC; Harold L. Hintz, Covington, Va.

[73] Assignee: Westvaco Corporation, New York, NY.

[22] Filed: Apr. 29, 1970 [21] Appl. No.: 33,044

[56] References Cited UNITED STATES PATENTS 3,532,596 10/1970 Bills etal. ..l62/70 Primary Examiner-S. Leon Bashore AssistantExaminer-Frederick Frei Attorney--Ernest B. Lipscomb and Robert S.Grimshaw [57] ABSTRACT A process for increasing pulp yield fromlignocellulosic material consists of treating the lignocellulosicmaterial with either a solution of cyanide ions at a temperature belowabout 130 C., preferably about 75 C., or treating the material with amixture of cyanide-bisulfite ions in solution at a temperature below 130C. preferably about 100 C., and digesting at conventional neutral oracid bisulfite conditions. The treatment of wood prior to neutral oracid bisulfite digestion with either cyanide ion alone or a combinationof cyanide-bisulfite results in pulps having increased yield andimproved brightness stability over untreated bisulfite cooks.

6 Claims, No Drawings A METHOD FOR INCREASING BISULFITE PULP YIELDCOMPRISING TREATING LIGNOCELLULOSIC MATERIAL WITH CYANIDE IONS AT A PI'IOF 7l2 PRIOR TO DIGESTION BACKGROUND OF THE INVENTION This inventionrelates to a process for increasing the yield of neutral or acidbisulfite pulps. More particularly, this invention is directed to aprocess for improving pulp yield of neutral or acid bisulfite pulp byreacting lignocellulosic material with either cyanide ions alone orcyanide ions in combination with bisulfite prior to digestion tostabilize the lignocellulosic material from degradation during pulping.

Lignocellulosic materials (such as wood from coniferous and deciduousspecies of trees, the stalks of bamboo and reeds, bagasse, and cottonfibers) principally contain lignin, cellulose, and hemicelluloses. Thelignin and hemicelluloses are located both in the cell wall of the fiberand in the inter fiber spaces, generally termed the middle lamellae. Inchemical pulping processes utilizing lignocellulosic materials, ligninin the inter-fiber spaces must be dissolved, with as little damage aspossible to the fibers, so that the fibers are readily separable.I-Iemicelluloses can be retained on the surface of the fiber, in whichcase they can influence the bonding properties of the fibers, and theyare also retained in the cell wall of the fiber, improving theflexibility and thereby the papermaking properties of the pulp. It isconsequently generally desirable to retain the hemicelluloses in orderto produce paper products having superior strength characteristics. Inacid and neutral bisulfite pulping, liquors directed toward dissolvinglignin attack the cellulose very little, but hemicelluloses are attackedto a high degree, especially when the cook is greatly prolonged in orderto remove as much of the lignin as possible before bleaching.

In attempting to prevent degradation of the cellulosic andhemicellulosic components, it was found that the terminal carbohydrategroup may be changed by converting the aldehyde end group to acarboxylic unit. The selected reaction for doing so is the well knowncyanohydrin synthesis or Kiliani-Fischer reaction for increasing thelength of the carbon chain of the sugars by addition of hydrocyanic acidto aldehyde or ketones, as follows:

The original procedure devised by Kiliani for the sugars required addinga sugar to an aqueous solution of hydrogen cyanide in the presence of alittle ammonia. A study of the effect of pH on the addition of cyanideto sugars was reported by Noji [52 Nippon Yakurigaku Zasshi 271-96(1956)]. No cyanohydrin formation occurred with both glucose andfructose at pH 5.45-2.65 In a modification of the cyanide addition, Nojicombined the cyanide solution with bisulfite addition and reported thatbisulfite adds more readily than cyanide to sugars and then cyanide maymore easily displace the bisulfite from the addition product.

Application of the Kiliani-Fischer reaction to treatment oflignocellulose material prior to alkaline pulping has resulted in US.Pat. No. 3,532,596 which issued Oct. 6, 1970.

This invention depends upon preventing degradation of the cellulosic andhemicellulosic components of bisulfite pulp. It is further successfulbecause the pulping solution is able to more effectively contact thelignin. Consequently, a lower concentration of pulping solution caninitially be mixed with the lignocellulosic material or a higherconcentration of pulping solution is present when steady cookingtemperatures are reached. The term yield as used herein is the dryweight of washed fibers which are recovered after pulping as apercentage of the weight of dry lignocellulosic material which wascharged to the digestion process. Any decrease in yield caused by lossof lignocellulosic material is generally undesirable in papermaking.

It is the principal object of this invention to provide a process forincreasing the yield in acid and neutral bisulfite pulps. A furtherobject is to produce a pulp having good brightness stability whilemaintaining increased yield without substantially impairing otherdesirable properties. Other objects will be evident from the followingdisclosure.

SUMMARY OF THE INVENTION It has been found that pulp yield producedunder acid and neutral bisulfite pulping conditions may be increased bytreating the lignocellulosic material, e.g., wood chips, with a solutionof cyanide ions at a temperature below about 130 C. and preferably belowabout 75 C., withdrawing the excess cyanide solution and digesting withbisulfite liquor under normal digesting conditions, or treating thechips with a mixture of cyanide-bisulfite ions in solution at atemperature below 130 C. preferably about C., until the reaction iscomplete before raising the temperature to digesting conditions. It wasfound the treatment of chips prior to digestion with both solutions ofcyanide ions alone or a combination of cyanidebisulfite ions results inyield increases in acid and neutral bisulfite cooks over the untreatedacid and neutral bisulfite cooks. Additionally, brightness stability isgreater for cyanide treated unbleached pulps than for untreated pulps.

DETAILED DESCRIPTION OF THE INVENTION The process of this inventioncomprises the steps of treating lignocellulosic material at a pH between7.0 and 12.0, with a solution of cyanide ions and hydroxyl ions theforming of cyanohydrin groups by reaction of the cyanide ion withaldehyde end groups of the lignocellulosic material and hydrolyzing thecyano portion of the thus formed cyanohydrin group into a carboxyl groupwhereby the lignocellulosic material is protected against degradationduring acid and neutral bisulfite pulping. To maintain satisfactorycontact between the cyanide solution and the lignocellulosic material, asolution to material ratio of 4 to l is preferred but is not critical aslong as sufficient contact is made.

Cyanohydrin is formed more rapidly at higher temperatures, buthydrolysis of cyanide ion directly into ammonia is also accelerated athigher temperatures. To avoid competition between these reactions forthe available cyanide ion, contact preferably occurs at relatively lowtemperatures such as about 75 C. However, the reaction to formcyanohydrin can take place at higher temperatures if desired and thereis no bisulfite present to compete with the reaction. To achieve themost favorable treatment conditions, the cyanide ions should becontacted at a concentration of 0.3 grams of cyanide ions per liter ofthe alkaline solution. As a practical digestion procedure for woodchips, diffusion of cyanide throughout the cellulosic material andcyanohydrin formation generally proceed simultaneously. If wood chipsare presteamed to drive out air and thoroughly wet the wood structure,about 70 percent of the possible reaction occurs in three hours at atemperature of 95 C. At a temperature of C., approximately 5 minutes issufficient, but control and uniformity of cooking conditions are notreliable. An economic penalty must also be paid for cyanide ion whichbecomes unavailable for cyanohydrin formation because of hydrolysisdirectly into ammonia. When cyanide alone is used, the hydrolysis steptakes place when contact is made with the bisulfite pulping liquor.

As another embodiment of this process, it is quite practical to includebisulfite with the cyanide solution to immediately form the hydrolysisreaction. In such cases the aqueous alkaline cyanide ion containingsolution should have at least 2.0 grams of bisulfite per liter.preferably sodium bisulfite for the most favorable reaction.

In the practice of this invention, any aqueous solution containingcyanide ions may be used. When KCN, for example, is added to water, itreadily dissociates into K" ion and CN ion. The cyanide ion then reactswith water:

The treatment conditions consisted of treating the chips with a mixtureof cyanide-bisulfite at 100 C. before subsequently raising thetemperature to 170 C.; or treating the chips with cyanide alone at 75C., draining the excess cyanide CN H O HCN OH and addition of bisulfiteliquor followed by subsequent cook- Whether derived from l-lCN gas,liquid hydrogen cyanide, an ing at 170 C. The cyanide solution was madeby dissolving 2.5 inorganic compound or an organic compound, anytreatment grams of potassium cyanide in 1 liter of water, as aconvenient solution therefore contains reactive CN ions, so the sourceof source of cyanide ions and was used at a 4:1 liquor-to-chip thecyanide ion is immaterial. ratio. The cyanide bisulfite solutions weremade by adding This invention may be practiced in continuous or bat h16.6 grams of sodium bisulfite to acyanide solution having 2.5 digesteroperations. A typical process for carrying out the ingrams of potassiumcyanide dissolved therein per liter. The vention, which is suitable forbatch or continuous operations, same liquor to chip ratio was used. Thetotal pulp yields were is: (a) presteam chips to drive out air, softenwood, and fill then determined and the results are shown in the table.

Type of treatment Untreated neu- Cyanide Cyanide Untreated acid tralbisulfite treated treated neutral Cyanide bisulfite bisulfite pulp pulpacid pulp pulp treated neutral pulp Cook number 1 2 3 4 5 6 7 8 9 10 1112 13 14 15 16 Cyanide consumed (percent on 0.1). wood) 3.0 3.0 3.0 3.03.0 3. 0 3. O 3. 0 3.0 Tinu to maximum temperature, min l l, 25 25 30 2520 3O 30 30 25 30 25 25 25 Tinlt zit-170 (3., min A. 120 00 (i0 30 120no 30 120 30 120 90 30 120 90 60 30 pllutbeginningofcook. H. 13.5! (5..14.1) 4.1 6.7 6.7 6.7 6.7 6.7 6.7 111] utvmlol'mok. 3.0 3.1 3.3 3.4 6.86.8 (5,8 5,3 5.4 7.0 (1.4 6.4 7.1! 7.3 7.4 7.3 'lotul pulp yivld(perm-111.011 0.1 wnml) 51. .2 54. 5 62.3 71.3 77. 7 82.7 84. 1 79. 785. 0 81.4 81.0 811. 2 80.11 8 1.7 85.0 83.2 Total lignin (percent on0.1). wootl).1l 13.8 18.13 22.1 11).) 21.6 22.5 21.5 22,6 18.5 18.1 22.3.r 19.6

pores with liquid; (b) introduce a solution of sodium cyanide at aconvenient concentration and at a temperature below 300 C. for atreatment which varies inversely in duration with the temperatures; (c)withdraw unreacted cyanide solution; and (d) introduce cooking liquor ata pH below 7 and digest at conventional bisulfite conditions.

It is therefore an advantage of this process that treatment oflignocellulosic material with cyanide ions gives increases in pulpyields when subsequently digested by the bisulfite process at neutral oracid conditions. Another advantage in producing unbleached pulp fromlignocellulosic materials in accordance. with this invention, inaddition to increased yield, is the brightness stability of theresulting pulp.

The practice of this invention may clearly be seen in the followingexample.

Pulping cooks were carried out in a multiple unit rotating digesterusing loblolly pine chips which had been screened to reject all chipspassing the VB inch screen and all retained by the 1 inch screen. Thecooks were designed to determine the effect of treatment of the chipswith cyanide and cyanidebisulfite mixtures on the pulp and paperproperties of the pulps resulting from acid and neutral bisulfitecooking of the chips. Several comparison cooks were made using a normalacid (pl-l 4) bisulfite process and also a neutral (pl-l 6.5) bisulfiteprocess. The neutral pH was maintained using a phosphate buffer. Thebisulfite cooking liquor contained 15 percent effective bisulfite(applied Nal-ISO as S0 on 0D. wood).

ln a typical cook, chips were placed in a filter flask and evacuatedwith a vacuum pump for 5 minutes. In the treatment cooks, the solutionwas admitted through a separatory funnel and the mixture was furtherevacuated for 15 minutes. The mixture was transferred to the digester,heated to the treatment temperature and held there for the prescribedtime. The treatment liquor was withdrawn after treatment, the cookingliquor at a 4:1 liquor-to-solids ratio added and the temperature raisedto 170 C. After the time at the cooking temperature was completed, thedigester was immediately cooled. The pulps were fiberized by a singlepass through the laboratory single disc Bauer refiner with the plateclearance set at 0.031 inch. After washing, the pulp was pressed toremove excess water and mixed in a Hobart mixer. Samples were taken todetermine the oven-dry weight of pulp for the total pulp yielddetermination. The remaining pulp was screened in the laboratory flatscreen with a 14 cut screen. The rejects were collected, dried at 105C., and weighed. The accepts were pressed, mixed and samples wereremoved for oven-dry weight determination.

The results show that the pulp yield of the four acid hisulfite cooks ofnon-treated pulp [cooks 1-4] ranged from 51.2 to 71.3 percent dependingupon the length of cooling time which varied from 2 hours to 30 minutes.The yield of acid bisulfite pulp cooks [cooks 8-9] treated .with cyanidealone varied from 79.7 percent at 2 hours to 85.0 percent at 30 minuteswhich is a substantial increase in yield. Yields of cyanide treatedneutral pulps showed 4-5 percent increases. The treatment of chips witha combination of cyanide-bisulfite results in yield increases of 2-3percent in the neutral bisulfite cooks over the untreated neutralbisulfite cooks.

The initial brightness of cyanide treated pulps was lower than thenormal acid and neutral bisulfite pulps. However, the brightnessstability after 2 hours heating at C. was much greater for the pulpsfrom the cyanide pretreated chips. The pulps from the untreated acid andneutral cooks lost approximately 45 GE. brightness units; whereas theboth cyanide treated pulps and the cyanide bisulfite treated pulps lostonly 1-1.5 G.E. units. Also, the appearance of the pulps was verydifferent. The normal acid bisulfite pulps had the familiar reddishappearance normally associated with sulfite pulps. The neutral bisulfitepulps were very pale yellow colored immediately following the cook, butquickly reverted to a medium brown color. The pulps from the cyanidetreated chips were initially a light brown color and remain so uponheating.

While the invention has been described and illustrated herein byreferences to various specific materials, procedures and examples, it isunderstood that the invention is not restricted to the particularmaterials, combinations of materials, and procedures selected for thatpurpose. Numerous variations of such details can be employed, as will beappreciated by those skilled in the art. below C. to convert theterminal carbohydrate unit of the lignocellulose chain to cyanohydringroup, withdrawing the excess solution, and digesting the thus treatedlignocellulosic material with a bisulfite pulping solution having a pHbelow 7 whereby the cyano portion of the cyanohydrin group is hydrolyzedto a carboxyl group.

2. The process of claim 1 wherein the lignocellulosic material is in theform of wood chips.

3. The process of claim 1 wherein said treating is conducted at atemperature about 75 C.

We claim cyanide ions at a pH between 7.0-12.0 and a temperature also 4.The process of claim 1 wherein said cyanide ion is obtained frompotassium cyanide.

5. The process of claim 1 wherein said cyanide ion is at a concentrationof at least 0.3 gram per liter in said solution,

mg? UNETED STATES PATENT QFFICE @ER'HNCATE GE RRETWN Patent No. 63,358Dated May 16, 1972 Inventor(s) Sten I. Falkehag et al It is certifiedthat error appears in the aboveidentified patent and that said LettersPatent are hereby corrected as shown below:

Column 3, In the Table, last line thereof, delete "M".

Column 4, lines 60-69 beginning with "below l30C" and concluding with"temperature about 7 C." are misplaced and should follow line 75 ofcolumn 4.

Signed and sealed this ZLLth day of Qotober 1972.

(SEAL) Attest:

EDWARD MQFLETCHERJ'R, ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents

2. The process of claim 1 wherein the lignocellulosic material is in theform of wood chips.
 3. The process of claim 1 wherein said treating isconducted at a temperature about 75* C.
 4. The process of claim 1wherein said cyanide ion is obtained from potassium cyanide.
 5. Theprocess of claim 1 wherein said cyanide ion is at a concentration of atleast 0.3 gram per liter in said solution, said solution to saidlignocellulosic material being at least 4 to 1 by weight, and saidlignocellulosic material is treated for at least 5 minutes.
 6. The pulpproduced according to the process of claim 1.